Check shape and remove deprecated APIs in scheduling_ddpm_flax.py

[ppham27]

model_output.shape may only have rank 1.

There are warnings related to use of random keys.

tests/schedulers/test_scheduler_flax.py: 13 warnings
  /Users/phillypham/diffusers/src/diffusers/schedulers/scheduling_ddpm_flax.py:268: FutureWarning: normal accepts a single key, but was given a key array of shape (1, 2) != (). Use jax.vmap for batching. In a future JAX version, this will be an error.
    noise = jax.random.normal(split_key, shape=model_output.shape, dtype=self.dtype)

tests/schedulers/test_scheduler_flax.py::FlaxDDPMSchedulerTest::test_betas
  /Users/phillypham/virtualenv/diffusers/lib/python3.9/site-packages/jax/_src/random.py:731: FutureWarning: uniform accepts a single key, but was given a key array of shape (1,) != (). Use jax.vmap for batching. In a future JAX version, this will be an error.
    u = uniform(key, shape, dtype, lo, hi)  # type: ignore[arg-type]

What does this PR do?

Fixes # (issue)

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[pcuenca]

Thanks a lot @ppham27!